An electret is a raw material which semi-permanently holds electric polarization in the inside thereof even in a state where an electric field does not exist in the outside thereof, thereby forming an electric field (exerting an electric force) in the outside, and has referred to one obtained by thermally or electrically treating a polymer material or an inorganic material which conventionally hardly conducts electricity, thereby semi-permanently polarizing a part of the material (being electrostatically charged or holding charges).
Electrets composed of a polymer material have hitherto been used in various forms including a film, a sheet, a fiber, a non-woven fabric, and the like depending upon a use embodiment thereof. In particular, electret filters obtained by molding processing of an electret have been widely used for an application of an air filter capable of efficiently adsorbing a fine dust, an allergen, etc. by the electric field, or other applications. Also, in the electrets, the utilization for various applications as a material for electro-mechanical energy conversion, such as a speaker, a headphone, a microphone, an ultrasonic sensor, a pressure sensor, an acceleration sensor, an oscillation controlling apparatus, etc., has become widened.
An electret using a porous resin film is known to exhibit a piezoelectric effect and can be used for sound detection, sound generation, oscillation measurement, oscillation control, etc. As for such an electret using a porous resin film, there are proposed applications to an oscillator of audio equipment, such as a speaker, a headphone, a microphone, etc., a pressure sensor in a flexible sheet form, and the like while utilizing its light weight (Patent Document 1).
Also, it is said that when a porous resin film is expanded in the thickness direction by using a high-pressure gas, an electret using this is enhanced regarding performances as a piezoelectric element (Non-Patent Document 1).
In the light of the above, as a method for expanding a porous resin film in the thickness direction, there is proposed a method in which a film having pores in the inside thereof is previously prepared by biaxial stretching, into which is then penetrated a high-pressure gas, and subsequently, the film is thermally treated under reduced pressure to obtain a porous resin film with a high expansion ratio (Patent Document 2).
It was considered that by allowing such a porous resin film with a high expansion ratio to hold a larger amount of charges in pores in the inside thereof, it becomes possible to obtain an electret having excellent performances and stability.
However, although in the techniques seen in the above-described academic document and Patent Document 2, the high-pressure gas introduced into the pores of the porous resin film enlarges the pores under reduced pressure to increase the expansion ratio; when the film is allowed to stand as is, it easily returns into the original state. Therefore, it is necessary to thermally treat the film in its expanded state, thereby accelerating crystallization of the thermoplastic resin to fix the shape.
Nevertheless, in such an expanded film, since the gas comes out gradually under reduced pressure, it was difficult to keep the film at a fixed porosity. Also, when the thermal treatment temperature is excessively elevated, the gas permeability of the thermoplastic resin increases, and moreover, the internal gas easily comes out, whereby the expansion ratio is lowered. Thus, the thermal treatment cannot be sufficiently carried out. As a result, there was a problem that it is difficult to obtain a desired porosity.
Furthermore, such an expanded film involved such drawbacks that it easily collapses in the thickness direction; that its energy conversion efficiency is lowered depending upon the environment at the time of use; and that it is unsuitable as an electro-mechanical energy conversion material for converting mechanical energy into electric energy.